1. Field of the Invention
The present invention involves an ultrasound Doppler method that permits non-invasive diagnosis and non-invasive unattended, continuous monitoring of vascular blood flow for medical applications.
2. Brief Description of the Background Art
Acoustic Doppler blood velocity measurement is a known medical diagnostic tool. The phased array steering of the acoustic beam and the phased array listening for the Doppler frequency-shifted echo are techniques that derive from a large body of work in the field of phased-array radar systems. The Doppler frequency shifts result from reflection of the transmitted acoustic beam from the moving blood constituents and are related in a known way to the velocity of blood flows. However, blood velocity monitoring is not currently practical for intensive care unit (ICU) of surgical applications. For non-invasive brain blood velocity monitoring, for example, a transcranial Doppler (TCD) probe must be mounted in a ball joint that is attached to the head by a helmet. The probe must be carefully aimed and fastened in place by an experienced person who knows how to locate the middle cerebral artery. Slight movements cause the probe to lose the blood velocity signal. Moreover, conventional Doppler ultrasound probes used in these devices scan (either mechanically or by using an acoustic phased array) in only one angle (which we will call azimuth), and will map only a single slice of the object being imaged. Efforts have been made to modify such devices to provide real-time three dimensional (3-D) imaging. However, in order for a two dimensional (2-D) device to provide such imaging normally requires thousands of elements, and must form many thousands of pencil beams every {fraction (1/30)} second. Sensor cost grows with the number of elements in the array and the number of processing channels. Thus, such devices are cost prohibitive, as well as impractical.
Moreover, no automated procedure exists in current practice for precisely locating the optimum point at which to measure the Doppler signal. Conventional ultrasound Doppler-imaging devices can only measure radial velocity in blood vessels, i.e., the velocity component parallel to the ultrasound wave direction, and not the vector velocity parallel to the blood vessel or the magnitude of the velocity of the blood through the vessel. Accordingly, what is needed is a new and useful Doppler ultrasound device method that can automatically locate the optimum point at which to measure the Doppler signal, and thus provide medical providers with parameters such as vector velocity, the volume of blood passing through the blood vessel and the Doppler spectral distribution of the blood flow and make those measurements over a large field of view for a single probe placement.
Copending applications PCT/US00/14691 and PCT/US00/16535 disclose a method of determining parameters of blood flow, such as vector velocity, blood flow volume, and Doppler spectral distribution, using sonic energy (ultrasound) and a novel thinned array. Also provided is a novel method of tracking blood flow and generating a three dimensional image of blood vessel of interest that has much greater resolution than images produced using heretofore known ultrasound devices and methods. The second of the above referenced applications discloses a novel probe geometry that offers a wide field of view. That geometry is referred to as a xe2x80x9cthinned arrayxe2x80x9d of transducer elements. Broadly, the present invention discloses an improved probe geometry permitting high-resolution imaging of a large volume of the subject""s body. In this improved geometry, the array elements are non-uniform in size and spacing.
A phased array will be referred to as thinned if its elements are spaced more than one half wavelength between centers. It will be referred to as sparse (or not filled) if there is space between the elements. A non-sparse (or filled) thinned array therefore has directive elements whose size is equal to the spacing between centers. Such an array is well behaved when focused at infinity and steered to broadside. This is because such an array becomes a continuous (no space between elements), uniformly illuminated, aperture. If the elements were narrow-band and omni-directional, the grating lobes, due to a path-length change corresponding to a 2xcfx80 phase shift, would be true ambiguities, indistinguishable from the main lobe or desired focus. When a filled, thinned array of uniformly-spaced equal-sized elements is uniformly illuminated (no element-to-element delays or phase shifts), the nulls of the element pattern coincide with the ambiguities of the far-field array pattern. The non ambiguous element pattern multiplies the ambiguous array pattern to reduce the grating lobes. See Patent Application No. PCT/US00/16535. When the array is either steered away from broadside or focused in the near field, ambiguities, called grating lobes, begin to appear. For example, if the array is steered or focused to the right, the desired beam is attenuated because it moves to the right, away from the peak of the element pattern and, more importantly, the nearest grating lobe on the left begins to move toward the peak of the element pattern. This limits the angular field of view of a thinned phased array, even if it is filled.